The present invention relates generally to an AC-DC converter and, more specifically, to an AC-DC converter that produces a DC signal with small ripples.
FIG. 1A illustrates a typical AC-DC converter, which includes a power conversion unit, an inductor-capacitor low-pass filter, and an input pulse generator. The power conversion unit includes a transformer with a primary and a secondary winding and two rectifier diodes with their anodes coupled to either prongs of the secondary winding respectively. The cathodes of the diodes are, in turn, connected to each other as well as to the low-pass filter. The primary winding is connected to the input pulse generator.
In operation, the input pulse generator outputs an AC signal, illustrated in FIG. 1B, across the primary winding. The signal across the primary winding is then transferred to the secondary winding, where negative portion of the signal is rectified by the rectifying diodes to yield the signal illustrated in FIG. 1C. This signal is then filtered by the low-pass filter to output a DC output signal that includes output ripples, as depicted in FIG. 1E.
Although output ripples are only small perturbations when output DC voltage level is high, the ripples become a prominent feature when the output DC voltage level is low. Since a variety of modem semiconductor devices such as microprocessors typically operate at low DC voltage levels, the ripples may interfere with the proper operation of the devices. Typical modem semiconductor devices have a tolerable ripple limit of about 1% of the DC voltage. Voltage ripple is of increased concern for semiconductor devices that operate at lower DC voltages, for example 1 V, because the absolute tolerable ripple decreases as the operating voltage decreases.
One way to minimize output ripples is by improving filtering capabilities of the converter. This may be achieved by increasing the capacitance and/or lowering the impedance of the capacitor in the low-pass filter. Real capacitors having a low series impedance or typically expensive. However, large capacitors take up more space and low impedance capacitors are expensive. Alternatively, a larger inductor may be used to improve filtering capabilities. However, a larger inductor also takes up more space. In addition, they saturate much easier.
Another way to reduce output ripples is to increase the frequency of the input signal. However, high frequency current flowing through the inductor produces high frequency flux change in the inductor ferrite, which increases core loss and decreases efficiency of the inductor. In addition, inductors with low loss ferrite material is expensive.
Some work has been done to reduce output ripples by modifying the typical AC-DC converter circuit. For example, U.S. Pat. No. 5,668,464 to Krein et al., which is incorporated herein, claims an AC-DC converter circuit that incorporates a feedback control circuit that generates an AC ripple signal to cancel out output ripples. Drawbacks of this converter circuit are that the feedback control circuit not only adds complexity to the circuit but also takes up precious space within small semiconductor devices. In addition, the output inductor carries large AC current ripples that can degrade the inductor. Another patent, U.S. Pat. No. 5,663,876 to Newton et al., which is incorporated herein, describes a rectifier circuit with two output inductors that can produce a DC signal without any output ripples. However, this can only be achieved by using inductors with specific inductance values operating at a predetermined operating condition. In addition, current and voltage ripples across the two output inductors are large and can degrading their performance.
Therefore, there is a need for an improved AC-DC converter that produces a DC signal with low DC voltage level, small voltage and current ripples, but high DC output current without large capacitors and/or inductors, addition of complex circuits, or strict operating requirements.
It is therefore an object of the present invention to provide an AC-DC converter that produces a DC signal with low voltage level and small output ripples without large capacitors and/or inductors, addition of complex circuits, or strict operating requirements.
It is another object of the present invention to provide an AC-DC converter that produces a DC signal with low voltage level, small voltage and current ripples across the output inductor, but high DC output current without large capacitors and/or inductors, addition of complex circuits, or strict operating requirements.
Briefly, the present invention provides an AC-DC converter that includes a plurality of power conversion units that overlay their signals to generate a DC signal with low voltage level, small voltage and current ripples, but high DC current. In a preferred embodiment, the AC-DC converter comprises a plurality of power conversion units, an input pulse generator system, and a low pass filter. Each power conversion unit preferably includes a transformer with a primary and a secondary winding, a DC blocking capacitor connected to the primary winding, and a rectifier diode with its anode connected to the secondary winding. The cathodes of the rectifier diodes are connected to one another as well as to the low pass filter, thereby connecting each power conversion unit to each other as well as to the low pass filter. The low pass filter is preferably an inductor-capacitor low pass filter. The input pulse generator system preferably includes a plurality of input pulse generators, each connected to the DC blocking capacitor of a power conversion unit.
In operation, each input pulse generator preferably outputs a square wave input signal that is the same as but out of phase with signals generated by other generators, so that the signals overlap and at least one of the signals is high at any given moment. In one embodiment, the square wave signals are evenly phase-shifted with respect to one another. Each input signal is transmitted to a primary winding through a DC blocking capacitor, which filters away any DC biases in the signal, and is then transferred from the primary winding to the secondary winding. The signal at each secondary winding is then transmitted to a rectifying diode, which rectifies the negative portion of the signal. Each rectified signal is then overlaid to generate a DC signal with low DC voltage level, small voltage and current ripples, but high DC current. The DC signal is then filtered by the low pass filter to further reduce voltage and current ripples to generate the DC output signal.
In an alternative embodiment, the AC-DC converter includes the same power conversion unit and low pass filter as the converter described above but with a modified input pulse generator system. The modified input pulse generator system preferably includes a first, second, third, and fourth n-channel MOSFET arranged in a full bridge configuration, where the drains of the first and third MOSFET are connected to each other and to ground, the sources of the same MOSFETs are connected to the drains of the second and fourth MOSFET respectively, and the sources of the second and fourth MOSFETs are connected to a DC voltage source and a DC blocking capacitor. The DC blocking capacitor is also connected to a first and second power conversion unit, where the first and second primary winding of the first and second power conversion unit each connect to the DC blocking capacitor on one prong and to the sources of the first and third MOSFET respectively on the other prong. Preferably, a first, second, third, and fourth pulse generator are connected to the gate of the first, second, third, and fourth MOSFET respectively. The first and second pulse generators output square pulses to alternately switch the first and second MOSFETs on; that is, either the first or the second MOSFET is switched on, but never at the same time. A brief period during which both MOSFETs are switched off is inserted in between alternately switching of the MOSFETs to prevent connecting the DC voltage source to ground. By alternately switching the first and second MOSFET on, the first primary winding is alternately connected to the DC voltage source and ground, generating a signal across the winding.
The third and fourth pulse generators output the same signals as the first and second pulse generators respectively but phase-shifted by 180 degrees. This generates a signal across the second primary winding that is the same signal as that across the first primary winding but phase-shifted by 180 degrees. The signals across the first and second primary winding preferably stay high longer than they are low, so that the two signals overlap while they are high and at least one signal is high at any one moment. The signals at the primary windings are then transferred to the secondary windings and then to the rectifying diodes, where negative portions of the signals are rectified. The rectified signals are then overlaid to create a DC signal with low voltage level with small voltage and current ripples but high DC current. The overlaid DC signal is then filtered by the inductor-capacitor low pass filter to further minimize ripples before output to a load.